Local variable execution plan change.

Hi AllNot sure if I am missing something obvious here.......Who do these 2 queries generate 2 different execution plans? Query 1 generates an index seek with a lookupQuery 2 generates a clustered index scan. This is against the AdventureWorks2012 database. [code="sql"]select SalesOrderID, OrderQty, ProductIDfrom Sales.SalesOrderDetailwhere ProductID = 710[/code][code="sql"]declare @var int set @var = 710select SalesOrderID, OrderQty, ProductIDfrom Sales.SalesOrderDetailwhere ProductID = @var[/code]Thanks